The present invention relates to an ink jet printhead, comprising a line of nozzles arranged with a uniform first pitch in a line direction X, a plurality of parallel ink channels having an axial direction Y normal to said line direction X and arranged in groups within which they have a uniform second pitch, each ink channel being connected to one of said nozzles, and a plurality of actuators arranged in groups corresponding to those of the ink channels, each actuator being associated with one of the ink channels for pressurizing ink contained therein, thereby expelling an ink droplet through the associated nozzle.
In a conventional ink jet printhead, the pitch of the nozzles is identical to the pitch of the ink channels, and the actuators, e.g., piezoelectric actuators, which are arranged with the same pitch, are made of a one-piece block of piezoelectric material which is cut in order to separate the individual actuators. The ink channels for all the nozzles of the printhead are formed by cutting grooves into a one-piece channel plate.
The width of such a printhead in the line direction X is necessarily constrained in view of considerations related to the (differential) thermal expansion of the actuator block and the channel plate, especially in the case of a hot melt ink jet printhead, and in view of the yield in the manufacturing process. When the width of the printhead is increased and, consequently, the number of nozzles, ink channels and actuators is also increased, the likelihood that at least one of the nozzles, ink channels or actuators is defective, will increase in proportion to the number of nozzles, and when only one of these elements is defective, the printhead must be discarded as a whole, so that the manufacturing yield becomes unacceptably low.
Theoretically, it would be possible to increase the width of the printhead, in order to provide a printhead extending over the whole width of a page, by aligning a plurality of printhead elements with the above construction in the line direction, so that their nozzles form a continuous nozzle array or line with a uniform pitch. However, for a printhead with a resolution of 75 dpi, for example, the pitch of the nozzles, and consequently also the pitch of the ink channels and the actuators is only in the order of 0.3 mm, and the printhead elements would have to be butted against one another in order to provide a continuous nozzle line with uniform pitch. As a consequence, the actuators for the first and the last nozzle of an individual printhead element would have to be arranged in the immediate proximity to the respective end of the printhead element, and it turns out to be difficult to manufacture a printhead element with such a construction. Moreover, if the actuator blocks of the aligned printhead elements are butted against one another, thermal expansion or contraction of the various components could still present a problem.
EP-A-0 921 003 discloses a printhead of the type described above, wherein the nozzles are offset from the center lines of their respective ink channels in the X-direction in such a manner, that the second pitch of the ink channels and actuators becomes smaller than the first pitch of the nozzles. As a result, it is possible to provide a wide printhead composed of a plurality of printhead elements or “tiles” which are disposed side by side, so that their nozzles form a continuous line with uniform pitch, whereas a larger spacing exists between the last actuator of one printhead element and the first actuator of the next printhead element. However, since each nozzle is formed directly at the end of the corresponding ink channel, the offset of the nozzle is limited to one-half the width of an individual ink channel. Thus, for a printhead element with a given number of nozzles, the difference between the pitch of the nozzles and that of the ink channels and actuators can only be relatively small.
EP-A-0 755 791 discloses an ink jet printhead in which each nozzle is connected to its associated ink channel and actuator by a flow passage that is inclined relative to the nozzle axis in the X-Y-plane. Thus, by varying the angle of inclination of the flow passages, it is possible to arrange the ink channels and actuators with a pitch that is different from the pitch of the nozzles. Then, however, the length of the flow passages varies in accordance with their angle of inclination, and this may give rise to non-uniformities in the printed image, because the different lengths of the flow passages induce differences in the process of droplet generation.